The use of cathode ray tube based instruments, such as oscilloscopes, in analyzing internal combustion engine performance has become prevalent in recent years. The increased use of these instruments is in part due to the increasing complexity of the electronic portion of contemporary internal combustion engines and in part due to the increasing technical capabilities of the persons performing the analysis of internal combustion engines. The spectrum of uses for such instruments in analyzing internal combustion engines ranges from monitoring the timing of the electrical and mechanical functions of the engine to determine the coincidence of spark discharge and cylinder valve operation to measuring the multitude of electrical signals necessary for controlling today's internal combustion engine.
In normal use, a linear sweep signal is repetitively applied to the horizontal deflection plates, or coils, of the cathode ray tube of an oscilloscope. The signal to be displayed on the oscilloscope is applied to the vertical deflection plates, or coils, of the tube. The linear sweep, or ramp signal, provides the linear horizontal time scale for the signal to be displayed. The repetitive linear sweep, or ramp signal, commonly has a sawtooth waveform. That is, the linear sweep or ramp signal has a slow rise time and a fast fall time. To sustain linearity, the sawtooth waveform must rise to a fixed amplitude at a constant rate. While the linearity of the fall slope of an oscilloscope sawtooth wave is not important, the linearity of the rise slope is important since the rise slope forms the horizontal time scale of the oscilloscope. Further, the amplitude of the sawtooth waveform must remain constant regardless of the frequency of the sawtooth waveform.
Past attempts to create sawtooth waveform signals in oscilloscopes have utilized wave shaping circuitry and averaging techniques to convert analog signals into sawtooth signals having a fixed amplitude and a linear rise slope. While thusly generated sweep signals can be synchronized by a trigger pulse, the averaging nature of analog circuits requires several cycles after a trigger pulse is first produced before a fixed amplitude sawtooth sweep signal is developed, resulting in a delay between the first occurrence of a trigger pulse and the production of a reliable display.
The delay between trigger pulse and reliable display associated with analog circuit generation of sweep signals results in the display trailing both the initial trigger pulse and changes in the trigger pulse by several cycles. If adequate time is not allowed for the sweep signal to accurately correspond to the trigger pulses, oscilloscope displays will be in error. When such an instrument is used to analyze the performance of an internal combustion engine, the response delay requires the engine to be run for longer periods of time. This is undesirable for several reasons. It creates an undesirable battery drain if the engine is not running. It also requires more operator time, thus adding to the cost associated with analyzing the engine.
As will be readily appreciated from the foregoing discussion, there is a need for a sweep generator for cathode ray tube based instruments that rapidly and quickly generates an accurate triggerd sweep signal. In particular, there is a need for a triggered sweep generator for cathode ray tube based internal combustion engine analyzers that generate an accurate triggered sweep signal more rapidly than prior art analog based triggered sweep generators.